Associated with a trend toward miniaturization and high performance of electronic devices in recent years, in printed wiring boards, density growth and high integration of the wiring density is developed. Along with this, there is an increased demand for enhancement of reliability by increasing the heat resistance of laminates for wiring. In such an application, it is demanded to have an excellent heat resistance and also a low coefficient of thermal expansion.
A laminate for printed wiring board is generally formed by curing and integrally molding a resin composition containing an epoxy resin as a major ingredient with a glass woven fabric. In general, the epoxy resin is excellent in a balance among insulating properties, heat resistance, costs and the like. However, in order to comply with a demand for an enhancement of heat resistance accompanied with high-density mounting and multi-layer configuration of a printed wiring board in recent years, there is a limit in an increase of the heat resistance, inevitably. Furthermore, since the epoxy resin has a large coefficient of thermal expansion, it is attempted to reduce the thermal expansion by selecting an epoxy resin having an aromatic ring or filling an inorganic filler such as silica in a high density (see, for example, Patent Document 1).
In particular in recent years, in a package substrate for semiconductors, associated with miniaturization and thinning, a warpage resulting from a difference in the thermal expansion coefficient between a chip and a substrate is a large problem at the time of component mounting or package assembling. Therefore, it is required that the thermal expansion should be reduced. However, it is known that an increase of the filling amount of the inorganic filler causes a lowering of insulation reliability resulting from moisture absorption, an insufficient adhesive force between the resin composition layer and the wiring layer, and a failure of press molding.
Although a polybismaleimide resin which is widely used for high-density mounting or highly multi-layered laminate is very excellent in the heat resistance, it has high moisture absorption and a problem with the adhesion. Furthermore, as compared with an epoxy resin, the polybismaleimide resin requires a high temperature and a long time at the time of lamination, so that it has a drawback that the productivity is poor.
That is, in the case of the epoxy resin, it can be generally cured at a temperature of 180° C. or less, whereas in the case of laminating the polybismaleimide resin, a long-term treatment at a high temperature of 220° C. or higher is required. Also, a modified imide resin composition is improved in moisture resistance and adhesion (see, for example, Patent Document 2); however, since it is modified with a low-molecular weight compound containing a hydroxyl group and an epoxy group for the purpose of ensuring solubility in commonly-used solvent such as methyl ethyl ketone, the resulting modified imide resin is significantly inferior in the heat resistance to the polybismaleimide resin.